This invention relates to an oil pressure warning system for an outboard motor, particularly to an oil pressure warning system for detecting (determining) and warning of the occurrence of an abnormality in lubricant properties, such as excessive low pressure of engine oil in an internal combustion engine for an outboard motor for small boats.
A conventional oil pressure warning system for an outboard motor has an oil pressure switch, installed at an appropriate location of a hydraulic circuit of the internal combustion engine or of an oil pan, which generates an ON signal when the oil pressure drops below a predetermined operating point, and when the ON signal is generated, it warns the operator and controls the fuel injection amount and ignition timing so as to decrease the engine speed to a level under which the engine does not suffer from damages, such as sticking or wear due to metal-to-metal contact.
As illustrated in FIG. 11, the pressure of engine oil (lubricant) PO varies with the engine speed NE and the oil temperature TO. In the figure, a straight line indicated as xe2x80x9cTOLxe2x80x9d illustrates the characteristic of oil pressure under low oil temperature, while another straight line indicated as xe2x80x9cTOHxe2x80x9d shows that of under high oil temperature. As will be seen from the figure, the oil pressure PO decreases with decreasing engine speed NE.
In the conventional oil pressure warning system, a single oil pressure switch is used and generates an ON signal when the oil pressure drops below a predetermined point of operation (illustrated as xe2x80x9cPOxxe2x80x9d in the figure) to alarm the occurrence of an engine oil abnormality, i.e., insufficient oil pressure. However, even if the oil pressure falls below POx, the oil pressure is still sufficient in the hatched portion (below the engine speed NEx and above the high pressure characteristic TOH). Thus, the conventional oil pressure warning system can not detect the oil pressure abnormality in the low engine speed region.
When the amount of oil is, in fact, extremely insufficient due to leakage, insufficient replenishment, etc., prompt notification is necessary. However, the output of the oil pressure switch remains unchanged until the engine speed drops below a certain level for the reason mentioned above. On the other hand, assuming that the operating point of the oil pressure switch is shifted to a lower pressure, it would become impossible to detect the oil pressure abnormality at a high engine speed.
Further, as illustrated in the figure, the characteristics are different for different oil temperatures. Since the oil viscosity decreases with increasing oil temperature, the characteristic under high temperature is lower than that under low temperature when the engine speed NE is the same. Since, however, no attention is paid for the oil pressure relative to temperature in determining the operating point of the oil pressure switch in the conventional system, when the oil pressure drops due to the oil temperature increases, the detection and alarming may sometimes be made erroneously in the conventional system.
Thus, the conventional oil pressure warning system leaves much to be improved.
An object of the present invention is therefore to solve the aforesaid problems by providing an oil pressure warning system for outboard motor, which can detect and trigger an alarm for the occurrence of an abnormality in the oil pressure accurately under any engine speed and oil temperature, such that the engine is reliably prevented from being damaged.
For realizing this object, the invention provides a warning system for oil pressure supplied to an internal combustion engine installed in an outboard motor, comprising: a first oil pressure switch installed in the engine which generates an output when the oil pressure is less than or equal to a first predetermined oil pressure; a second oil pressure switch installed in the engine which generates an output when the oil pressure is less than or equal to a second predetermined oil pressure set higher than the first predetermined oil pressure; oil pressure abnormality determining means for determining whether the oil pressure is abnormal based on at least one of the outputs of the first and second pressure switches; and alarming means for alarming when the oil pressure is determined to be abnormal.